feat(theme): ship rainbow_color as default

[Cec1c]

Summary

• ship rainbow_color as the default CCU theme with the approved V3 palette
• migrate legacy CCU theme and status-line preferences while preserving custom theme choices
• package rainbow_color alongside the legacy Hermes theme
• update Manager preview, installer copy, uninstall ownership, tests, and release copy

Validation

• npm test (263/263)
• npm run build
• git diff --check